Does this break on Windows?

Paste Node.js code. It tells you which lines fail on Windows, what the error will look like, and what to write instead. Nothing to install.

Your code stays in your browser. The checker is plain JavaScript running on this page — there is no upload, no server, and no request made when you type. Turn off your network and it still works.

Try one

Waiting for code

What this cannot tell you

Being straight about it, because a checker that oversells itself is worse than none:

It reads text, not a syntax tree. It follows a variable one hop and no further.
A clean result means these patterns were not found. It does not mean your code runs on Windows. Path lengths, file locking, PATH resolution and antivirus only show up when the code actually runs.
It cannot see intent. Code that is deliberately Linux-only — a script generating a systemd unit, say — looks identical to code that forgot about Windows.

This box checks one snippet. The command-line version walks a whole repository and exits non-zero, so it drops into CI as one line. It is free and MIT licensed, forever.

The free CLI on GitHub Have me check the whole repo